The present invention relates to a process for producing a high-purity silica from a crude silica such as a compound or a mixture which is usable as a source for a silica.
There is an increasing demand for a high-purity silica as a functional material such as a material of quartz, a ceramic material and a filling material.
Conventionally, a process for producing a silica by neutralizing sodium silicate with an acid or an ion exchange resin (sodium silicate method) is known. This process, however, is not suitable as a process for producing a high-purity silica in spite of a low cost, because metallic impurities such as sodium, aluminum and iron are contained therein.
As a dry process, a process for producing a silica by combustion of silicon tetrachloride at a high temperature in the presence of oxygen and hydrogen is known. This process, however, restricted to a narrow range of use due to a high cost. In addition, the properties of the silica obtained by the dry process is different from those obtained by a wet process, the control of the properties of the silica is difficult.
Further, as a process for producing a silica from silicofluoride, are known a process for producing a hydrated silica and ammonium fluoride, which comprises heating a silica to a bright red heat, treating the silica with ammonium fluoride under heating thereby forming silicon fluoride, ammonium silicofluoride and/or silicon diamino tetrafluoride, volatilizing the silicon fluoride, ammonium silicofluoride and/or silicon diamino tetrafluoride thereby collecting them, and treating the collected material with water and ammonia [U.S. Pat. Nos. 1,859,998 and 1,959,749]; a process for producing a silica, which comprises dissolving a compound or a mixture which is usable as a source for a silica, in a mixed solution of hydrofluoric acid and sulfuric acid, distilling a fluoride of silicon from the resultant solution, and adding ammonium to the distillate [Japanese Patent Application Laid-Open (KOKAI) No. 62-153,111 (1987)]; and a process for producing a silica and ammonium fluoride, which comprises recovering silicon tetrafluoride-containing gas from the acidulation of a fluoride-containing phosphorus source such as a phosphorus-containing rock, absorbing the resultant gas in water or a solution of ammonium fluoride thereby obtaining an fluosilicic acid or ammonium fluosilicate solution, further converting the fluosilicic acid to ammonium fluoride in the case of fluosilicic acid solution, and ammoniating said ammonium fluosilicate solution [European Patent Application Publication No. 0,337,712].
The process disclosed in U.S. Pat. Nos. 1,859,998 and U.S. Pat. Nos. 1,959,749 is complicated and has a problem in selection of materials constituting an apparatus due to the high volatilizing temperature (about 300.degree. C.), and therefore this process cannot be said to be a preferred industrial process. The process disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 62-153,111 cannot be said to be an industrial process because it uses hydrofluric acid which costs much, and by using a considerably large amount of sulfuric acid, a waste acid is produced as a by-product. The process disclosed in European Patent Application Publication No. 0,337,712 has no industrial merit in case that it is not combined to a manufacturing plant for a wet phosphoric acid process. Also, the process of purification in gaseous state has a problem of a loss in absorption step of a gas and of a trouble in the operation by scaling of silica, and further a high cost of the apparatus.
As a result of studies undertaken by the present inventors so as to solve these problems, it has been found that by producing ammonium silicofluoride from a crude silica and ammonium fluoride, acid ammonium fluoride of a mixture thereof, separating the impurities from the reaction product by utilizing the difference in solubility, and reacting the thus-obtained purified ammonium silicofluoride with ammonia in an aqueous medium, a high-purity silica is obtained. The present invention has been accomplished on the basis of this finding.